Remove add_generator

include/libsemigroups/presentation.hpp

@@ -440,24 +440,6 @@ namespace libsemigroups {
     //! \throws LibsemigroupsException if \p x is in `p.alphabet()`.
     void add_generator(letter_type x);
 
-    //! \brief Return whether \p x is a generator.
-    //!
-    //! Returns \c true if \p x is a generator (i.e. is contained in the
-    //! alphabet) and \c false otherwise.
-    //!
-    //! \param x the letter to check.
-    //!
-    //! \returns A value of type `bool`.
-    //!
-    //! \exceptions
-    //! \no_libsemigroups_except
-    //!
-    //! \complexity
-    //! Logarithmic in the size of the alphabet.
-    [[nodiscard]] bool contains_generator(letter_type x) const {
-      return _alphabet_map.find(x) != _alphabet_map.cend();
-    }
-
     //! \brief Return whether the empty word is a valid relation word.
     //!
     //! Returns \c true if the empty word is a valid relation word, and \c false

include/libsemigroups/presentation.tpp

@@ -168,7 +168,7 @@ namespace libsemigroups {
   template <typename Word>
   void Presentation<Word>::add_generator(
       typename Presentation<Word>::letter_type x) {
-    if (!contains_generator(x)) {
+    if (!in_alphabet(x)) {
       add_generator_no_checks(x);
     } else {
       LIBSEMIGROUPS_EXCEPTION("the argument {} already belongs to the alphabet "